1. Field of the Invention
The present invention relates to a switching method for a BLSR (Bidirectional Line Switched Ring) and a node apparatus used in the ring, and, in particular, to a switching method for a four-fiber BLSR and a node apparatus used in a four-fiber BLSR.
2. Description of the Related Art
A BLSR is a ring network system in which one time slot in a line is used by a plurality of paths, and another time slot is had as a spare in common by the plurality of paths, and, thereby, high line holding efficiency can be achieved.
In a 4-fiber BLSR configuration, there are two methods of recovering from a fault condition on ring. A first method is span switch using a short path between nodes between which a fault exists. A second method is ring switch using a long path. When span switch could not be performed, ring switch using a long path is performed so that recovery from the fault condition is achieved.
According to BELLCORE standard GR-1230-CORE Issue 4, R6-151 for SONET (Synchronous Optical Network) BLSR equipment generic criteria, it is prescribed to perform ring switch by SF (Signal Fail) or SD (Signal Degrade) when a notice of reception cannot be received by a short path from an adjacent node within a predetermined time after span switch by serious SF (that is, SF-S) or span switch by slight SD (that is, SD-S) is performed.
FIG. 1 illustrates span switch.
In the figure, in a node A, when a fault in a working line Wba is detected, a span switch SS2a of the node A and a span bridge SB1b of a node B are switched from the working line Wba to a protection line Pba, and, also, a span bridge SB2a of the node A and a span switch SS1b of the node B are switched from a working line Wab to a protection line Pab. Thus, a span switch operation is performed.
FIG. 2 illustrates ring switch.
In FIG. 2, when a fault in the working line Wba and a protection line Pba is detected in the node A, a ring switch RS2a and a ring bridge RB1a of the node A are switched so that output from the node A to the working line Wab is connected to a protection line Paf, and, also, input from a protection line Pfa is connected to the working line Wba of the node A. Also, a ring switch RS1b and a ring bridge RB2b of the node B are switched so that input from a protection line Pcb is connected to input from the working line Wab of a node A, and output from the node B to the working line Wba is connected to a proportion line Pbc. Thus, a ring switch operation is performed.
It is assumed that a case occurs where upon occurrence of a fault, span switch cannot be performed and therefore ring switch is performed. Then, after that, even when recovery is made from the situation in which span switch cannot be performed, it is not possible to know this fact of recovery, and to know a time when a check should be made to determine whether or not the recovery is achieved.
Once ring switch is performed, recovery from the fault condition is achieved. Accordingly, it is not necessary to perform span switch, and it is not necessary to always make a check to determine whether or not recovery is made from the situation in which span switch cannot be performed.
However, it is necessary to make a check to determine whether or not recovery is achieved from the situation in which span switch cannot be perform, when a fault occurs in another span, or switching will then be made by the reason why recovery is achieved from the situation in which span switch cannot be perform.
However, because a check operation for such a case is not prescribed, there may be an apparatus in which recovery can be made from a fault condition and an apparatus in which apparently recovery cannot be made from a fault condition, although recovery can actually be made in either apparatus. Accordingly, compatibility is degraded.
Further, during execution of ring switch, as a result of a lately made switching request having a high priority being performed, the contents of K1 and K2 bytes for transmitting/receiving a switching protocol, that is, APS (Automatic Protection Switch) information is not stabilized in the APS of overhead of SONET. Thereby, a switching operation is repeated, and an alarm of APS occurs.